The floating state of a submersed submarine is produced by virtue of the fact that the buoyancy resulting at the desired diving depth is balanced by varying the weight. To raise the boat weight, water is taken up in this case in one or more containers of the submarine, so-called cells (flooding), while, to lower the boat weight, water is released to the outside from the cell(s) (pumped out). In this case, so-called regulating cells serve the purpose of coarse weight setting, while so-called deep pumping-out cells are provided for the fine balance. Consequently, the latter cells have a comparatively low volume, while the regulating cells can have a capacity of several hundred liters. The cross section of a pipe connection between a regulating cell and an opening in the boat's hull is appropriately large, so that a rapid variation in the filling level is possible in the relevant regulating cell. On the other hand, it is to be possible to set the filling level as exactly as possible in such a regulating cell, so that the volume of the additional deep pumping-out cells can be kept as small as possible. This requirement encounters the problem that a flap determining the beginning and end of the flooding or pumping-out operation of a regulating cell can be opened or closed in the above-named pipe connection only at a comparatively low speed of the order of magnitude of a few to approximately ten seconds. The respective actuating operation must therefore be begun early, particularly in the case of closure of this flap, long before the desired filling level is reached in the regulating cell. However, this lead time can only ever be determined with some degree of accuracy if the rate of flow in the relevant pipe connection to the regulating cell always has a known value, at least at the beginning of the closing operation, and is subjected as far as possible to no other fluctuations. It is then possible to precalculate, with some degree of accuracy, the volume of water presumably still flowing through by multiplying this flow rate value by the expected closure time of the flap as well as, if appropriate, a factor taking account of the variable position thereof, and thus to predict that filling level in the relevant regulating cell at which the closing operation of the flap must be initiated. On the other hand, the rate of flow in said pipe connection to the regulating cell is a function, in particular, of the pressure difference between the pressure inside the regulating cell and the outboard water pressure, and can therefore fluctuate strongly not only with the diving depth, but also, in particular, with the filling level inside the regulating cell. Thus, with the bleed valve closed the pressure in the regulating cell rises continuously during flooding, the outboard water pressure is substantially influenced, particularly in the case of low diving depths owing to the wave motion, etc., and so a multiplicity of factors act on the current rate of flow in the pipe connection to and from the regulating cell. Consequently, severe difficulties have been encountered to date in determining the correct instant to initiate the closing operation for the flap in the pipe connection from and to the regulating cell.